1. Field of the Invention
The present invention relates to a gas detecting element. More particularly, the invention relates to a gas detecting element specifically adapted for mass production and a process for producing such an element.
2. Description of the Prior Art
A previously known element used for detecting various gaseous components such as those in the exhaust gas generated by burning a fuel is constructed with a semiconductor wafer made of a transition-metal oxide and electrodes composed of a pair of noble metal wires one end of each of which is coupled to the wafer. To produce such detector element, a pair of noble metal bodies serving as electrodes are placed side-by-side in a mold and the mold is charged with a semiconductor material of a transition-metal oxide which is pressed into a desired element shape. With this technique, there is a difficulty in providing a product in which a space of not more than 0.1 mm between the electrodes is maintained at a high accuracy so that the electrical resistance of the element is less than 5 megohms so that a safety device inside the element can operate at low voltage. Also, the technique is unable to provide large area electrodes and thus is not advantageous for manufacturing gas detecting elements having a reduced electrical resistance.
The inventors of the present invention have previously proposed in Japanese patent application No. 40561/78 a gas detecting element constructed of a semiconductor wafer of a transition-metal oxide, platinum electrodes bonded to both sides of the wafer with electrodes connected to noble metal wires, and a porous ceramic protective layer covering the surfaces of the electrodes. When a gas detecting element of such construction is used in a normal operating temperature range for an air/fuel ratio detector for controlling the exhaust gas from an internal combustion engine, it provides an overlapping portion in the electrical resistance temperature characteristic curves even if it changes suddenly at the theoretical mixing ratio. The "overlapping portion" is defined in the commonly assigned former U.S. application Ser. No. 112,977 filed on Jan. 17, 1980. Thus, it is capable of achieving correct detection of the gases in the exhaust and thus providing information concerning the theoretical mixing ratio. However, the required method for manufacturing a detecting element of such sheet structure involves a complex step of burying metal lead wires. This has presented a serious problem that has yet to be solved before mass production of such elements can be implemented.